After you have selected "Custom RTMP" a new windows will open where you will input your Primcast provided streaming details.
In the "RTMP URL" field enter your streaming address which should look something like this: Rtmp://address.srfms.com:port/live
You will find your streaming address in your Primcast welcome email, or in your Primcast Dashboard.
In the "stream name" section enter your desired stream name, the most commonly used name is "livestream" but you can use any name you wish.
After doing so, you can select your encoding preferences like audio-video bitrate and codec.